1. Field of the Invention
The present invention concerns a method of controlling the power at which an access packet is transmitted by a mobile in a mobile radio system.
It also concerns a mobile radio system using this method.
2. Description of the Prior Art
Experience shows that in mobile radio systems the proportion of time for which a mobile is communicating with a base transceiver station is usually low. The mobile has to transmit only in the following situations:
during a call, whether this originates from the mobile or from the base transceiver station; PA1 during exchanges of signalling information between calls when a mobile signals its presence or its location. PA1 local attenuation, or fading according to a lognormal law, which depends on the propagation environment and the distance between the base station and the mobile, called hereinafter the "propagation loss"; PA1 Rayleigh fading caused by interference resulting from multiple propagation paths and fluctuating much faster than local attenuation. PA1 at the base transceiver station the broadcast channel transmits a power indication representing the transmit power of said channel; PA1 at the mobile the power received on said channel is measured, the measured power is compared with the transmit power indicated by the transmit power indication on the broadcast channel to deduce an estimated propagation loss from said mobile to said base transceiver station, and an optimal transmit power for said access packet is determined on the basis of the estimated propagation loss and parameters of said mobile radio system.
When any such period of activity is initialized the mobile must send a first message, called the access packet, without knowing in advance the attenuation with which it will arrive at the base station. The access packet is therefore transmitted at a power level dependent only on the maximal power rating of the mobile, possibly subject to a power limit broadcast by the base station for all mobiles in its coverage area.
Signal attenuation in a mobile radio system can be modelled by means of two components, as described in "Mobile Cellular Telecommunications Systems" by W. C. LEE, McGRAW-HILL International Editions, 1990:
The power of the signal received by the base station therefore varies considerably with the position of the mobile in the coverage area. The receiver of the base station must therefore be designed to have a very wide input dynamic range, which in practice raises major problems. Input dynamic ranges in excess of 100 dB are frequently used, for example. The downstream stages must sometimes be duplicated. Several systems with a smaller dynamic range are then connected in parallel to cover the overall requirement.
This very wide dynamic range is unfortunately needed only for the access packet, as the base station can estimate the level of the received signal and respond by indicating to the mobile the transmit power to use to achieve a high quality link without using unnecessarily high power.
An object of the present invention is to remedy these drawbacks by providing a method for controlling the power at which an access packet is transmitted by a mobile such that the input dynamic range of the base transceiver station receiver can be reduced.